As our society increasingly comes to depend upon electronics technology an almost ubiquitous problem is that the transducers which we use to detect and control actions in the physical world work primarily with analog signals, yet the systems which we use to process them work with digital signals. It follows that conversion between these signal types is often required, and analog to digital (A/D) converters and digital to analog (D/A) converters have become staple components used in modem electronics design. Of primary concern herein are our ability to rely upon such converters as themselves being accurate, and our ability to effectively use them in larger systems (herein termed "external systems," since they can be spoken of as employing converters within them).
Turning first to accuracy, many factors may prompt concern in this respect. Converters must be calibrated during manufacture, and when external systems are assembled it is usually also wise to calibrate the resulting system as a whole. Further, if for nothing other than that time and events have progressed, the users of such systems often wish to recalibrate them. Aside from being a valuable form of problem diagnosis, eliminating calibration as one possible problem in a divide and conquer approach, being able to do this often significantly affects end-user's confidence in the system. Still further, converters may operate in several operating modes, being switched back and forth between modes, and possibly having several input and output channels, as well. This all compounds the problems associated with calibration and recalibration.
There are also usually very pragmatic reasons for recalibrating. Probably the most significant of these are temperature related, but many other influences and their dynamics may also exist, like pressure, mechanical stress, non-thermal radiation, etc. As for temperature, the environment in which electronic components are called upon to operate may be very dynamic and range widely in temperature. Further, electronic components by their very nature are subject to heating as they operate, commonly called "temperature drift," and A/D and D/A converters are particularly susceptible to this.
Turning to converter utility, converter calibration and recalibration (henceforth collectively just "calibration") was typically a manual operation until very recently. Technicians would make adjustments to potentiometers ("pots") mounted on circuit boards, requiring delay until they could arrive on-site and disassemble equipment or otherwise obtain access to the pots. In some cases removing covers and such might even expose the converters to different ambient environmental conditions, making calibration particularly difficult. And once on-site, and once the particular model of converter was determined, the technician might have to refer to instructions for performing calibration, since there was no set protocol across the multitude of converter manufacturers and even across the product lines of some manufacturers. This all often resulted in considerable down-time for the external system, or at least a period of reduced reliability. And, of course, the problem of manual calibration was severely exacerbated when such converters were used in truly remote locations and extreme environments, like outer space, the ocean's depths, military battle fields, etc. For these and other reasons, manual calibration is an undesirable solution.
As one solution, converters can be designed which do not require frequent calibration. They can use ultra-stable components, ones which are relatively insensitive to temperature change. But using such throughout a system is usually prohibitively expensive. Alternatively, the expected accuracy of the converter and the external system using it can be lowered, but this is merely an acknowledgement of the existence of the problem, without an attempt to solve it.
Various alternatives to manual calibration have also been attempted. Modern programmable components can be employed that allow users to calibrate the internal circuits using software commands. Recent examples in this respect even permit the use of programs having or operating under sophisticated graphical user interfaces (GUIs). However, calibration using such programs usually requires intervention in the underlying process employing the converter, which may be a serious disruption in some of the applications where converters are widely employed today. Software calibration is also increasingly portrayed as a plug and play "autocalibration" feature which is "user friendly," but such advertising puffery often fails to live up to end-user's expectations. The lack of computer user sophistication and the need to provide software programs usable across multiple operating systems and hardware platforms are often further parts of the overall problem.
Existing automatic calibration using software or hardware commands are also usually based on the drift of the system's performance over long time periods, and are not made to respond to more rapidly occurring changes, such as temperature fluctuations, for one example. Although this type of software program approach has been termed "autocalibration," that term is perhaps not strictly accurate. When a user instructs a program to perform calibration the calibration is not completely automatic, since it has not occurred autonomously. However, in recent years, there have been attempts to allow circuits to perform true autocalibration.
U.S. Pat. No. 4,490,713 to Mrozowski discloses a microprocessor supervised A/D converter which compensates for offset drift errors due to temperature variation by using a system of recirculating remainder conversion. Generally, an initial analog input signal is converted to a digital value and stored, converted back to an analog signal which is summed with the original analog signal, and then reconverted and stored as a sixteen-bit digital word representing the analog input signal. During an initial calibration routine, full-scale positive and negative reference voltages are applied, processed, and stored as digital words. The output of a differential temperature sensor is similarly processed to produce a sixteen-bit temperature reference word and stored. During normal operation, the output of the temperature sensor is digitized and stored, and as the temperature changes a preprogrammed gain and offset storage register is searched for an appropriate compensation factor for the attendant temperature drift. The digitized word from the converted analog input is then compensated by this factor.
Although the invention in Mrozowski is an advance, there remain several disadvantages. The reliance on a preprogrammed register with pre-calculated compensation factors may produce a compensation which does not meet a present need. Because the compensation is applied digitally, there can be problems with the data range and missing codes. The process used also does not allow for a multimode circuit.
U.S. Pat. No. 5,319,370 to Signore discloses a system for calibration in an analog voltage reference used by an A/D converter. This system is termed "continuously calibrated" since it may continuously work to maintain its own compensation. This is accomplished by use of both an untrimmed analog reference voltage and a temperature measurement device. In one embodiment, the untrimmed analog reference voltage is converted to digital, compensated by applying a prestored digital trim command word, and converted to back to an analog voltage which is used to trim the analog multiplexer used by the A/D converter. An alternate embodiment uses the prestored digital trim command word to adjust the gain and offset in the digital output of the A/D converter.
This system, however, has a number of disadvantages. Neither embodiment provides an ability to the A/D converter for multiple input ranges, to recall the calibration factors in response to a change in input range, or to automatically determine when recalibration is necessary. This system also does not provide an ability to calibrate the circuit as a whole, which is particularly desirable for correcting temperature-based errors. The compensation used here is applied merely to the reference but not to other portions of the circuit.
From the preceding, it follows that there is a need for a truly automatic calibration system which responds intelligently based on key triggering stimuli, and which operates reliably across a plurality of modes, ranges, and channels.